This invention relates to zeolitic molecular sieve compositions characterized by outstanding capability to complex multivalent cations, especially calcium. In particular, the invention relates to zeolitic molecular sieve compositions, especially those based on molecular sieves having a high aluminum content, in which the zeolites are characterized by novel particulate morphology. The invention also relates to novel methods of forming zeolites to increase rates of zeolite crystallization, provide the novel morphology and provide a direct effect on sequestration performance.
With environmental concerns over phosphates rising during the last generation, zeolite molecular sieves have taken a dominant role as the water softening builder component of most detergents. Environmentally xe2x80x9cfriendlyxe2x80x9d, zeolites have been a poor substitute for phosphates from a performance standpoint, having both lower calcium and magnesium sequestration capacities as well as much lower rates of sequestration. The sequestration properties of zeolites arise from their ability to ion exchange. This ion-exchange ability derives from tetrahedral Al(III) inherent in classical zeolite frameworks. Each aluminum induces one negative charge on the framework which is counterbalanced by an exchangeable cationic charge. Thus, exchange capacity is limited by the aluminum content and xe2x80x9cdetergentxe2x80x9d zeolites have been restricted to the relatively short list of xe2x80x9chigh aluminumxe2x80x9d zeolites. By Lowenstein""s Rule, the Si/Al ratio of a zeolite may not be lower than 1.0 and concomitantly, the aluminum content may not exceed 7.0 meq per gram for an anhydrous material in the sodium form. This capacity may alternatively be expressed as 197 mg CaO per gram zeolite (anhydrous) when water softening is the desired exchange reaction. Zeolites demonstrating this maximum aluminum content include Zeolite A, high aluminum analogs of Zeolite X and high aluminum analogs of gismondine (often referred to as Zeolite B, P or MAP).
While Zeolite A has been the xe2x80x9cdetergent zeolitexe2x80x9d of choice for years, the possibility of employing a high aluminum version of gismondine-type materials in calcium sequestration has been known for more than a generation (U.S. Pat. No. 3,112,176 Haden et al.) and has recently found renewed interest (for example, U.S. Pat. No. 5,512,266 Brown, et al.). In addition to zeolites, the ability of silicates to complex ions such as calcium and especially magnesium has long been known and sodium silicate has long been employed as a cheap, low performance detergent builder. More recently, complex silicates such as Hoechst SKS-6 have been developed which are claimed to be competitive with higher performance zeolites.
The capacity for silicates to complex ions such as calcium and magnesium is inversely proportional to silicate chain length and directly proportional to the electronic charge on that chain fragment. Silicates depolymerize with increasing alkalinity. At moderate pH (where wash cycles are conducted) silicates are polymeric. However, at much higher pH""s silica not only becomes predominantly monomeric, but that monomer may possess multiple charges. If such small, highly charged silicate fragments could be exposed to solutions bearing multivalent cations, very powerful high capacity sequestration agents would result. In commonly assigned U.S. Pat. No. 5,948,383 (Kuznicki, et. al.) such a situation was created by isolating and stabilizing substantial concentrations of such charged silicate species within zeolite cages where ions such as calcium are free to enter from an aqueous environment (such as wash water) and react with these powerful sequestration agents. The zeolites of this patent have been characterized as hybrid zeolite-silica compositions (HZSC) which demonstrate unusual and beneficial properties in complexing multivalent cations. Such hybrid materials are prepared by crystallizing high aluminum zeolites in highly alkaline/high silica environments. Chemical analysis indicates an excess of silicate in these species beyond that inherent to their crystalline frameworks. Such materials demonstrate sequestration capacities for cations such as calcium which not only exceed the amount of zeolitic aluminum available for ion-exchange, but in fact, may exceed the theoretical limit possible for a zeolite. These materials and their properties must be considered something distinctly different than zeolitic. Such compositions show extreme promise as water softening agents and detergent builders and may find other applications in complexing multivalent cations such as in the removal of calcium from sugars and fatty oils or in removing heavy metal such as lead from various streams.
It is believed that the key mechanism in the effectiveness of these patented materials is derived from the ability of zeolite cages to isolate and stabilize much smaller, more highly charged silicate units than exist in normal aqueous solutions such as wash water. These silicate units are introduced during synthesis of said hybrid zeolite-silica compositions by providing an environment wherein silica in the reaction mixture is depolymerized to highly charged predominantly monomeric units before crystallization begins.
These occluded silicate units are readily visible in 29Si NMR spectra. Such units are much more powerful in complexing multivalent cations than existing silicate compositions used for that purpose. The zeolite framework and occluded silicate units act in concert, as a new type of hybrid composition, showing properties neither zeolites, silicates nor physical blends of the two demonstrate. In addition to high capacity, these new hybrid compositions demonstrate unusually rapid rates of sequestration, a critical parameter in applications such as detergent building.
It has now been discovered that the condition used to form the hybrid zeolite-silica compositions as described in U.S. Pat. No. 5,948,383, can yield novel zeolite particle morphologies. The zeolite particles which are formed are extremely rapid cationic sequestrants rendering the zeolite compositions useful, particularly as detergent builders. In the field of detergent building, rapid calcium sequestration is key to effective employment of surfactants. Commercial zeolite A suffers from slow exchange kinetics. This is especially true in the growing area of cold water detergents where the rate problem renders it essentially ineffective. While zeolite A, by definition, contains equimolar aluminum and silicon in its framework structure and thus the maximum possible zeolite ion-exchange capacity, useful ion-exchange capacity in most processes, however, is a dynamic function based on inherent capacity, ion selectively and kinetics of exchange. While zeolite A is a fixed composition with a fixed exchange capacity and fixed ion selectivities, the kinetics of exchange vary widely with the physical and morphological properties of zeolite A crystals and the aggregates into which they are formed. Microcrystalline zeolite A has long been known to improve exchange kinetics, but these submicron particles tend to stick to fabric in a wash cycle rendering such kinetically enhanced builders essentially unusable in real world detergent applications. Submicron microcrystals grown into macroscopic aggregates would solve this problem, yielding the kinetic advantages of microcrystalline exchangers and the handling/non-sticking properties of macroscopic ensembles.
It has now been found that uniform aggregates of submicron zeolite microcrystals can be formed by in situ processes where essentially all of the aggregrate material is between 1 and 5 microns. Surprisingly, even though such materials exist as macroscopic aggregates, the exchange kinetics are extraordinarily rapid, reflecting the inherent rate of the substituent submicron crystals. The advantages of maintaining the substituent crystals as coherent macroscopic aggregates is both in the ease of handling when used as a substituent in manufacturing compounds such as detergent mixtures, as well as in minimizing pressure drop in flow-through applications such as water purification/softening filters.
The enhanced properties of these macroscopic aggregates of submicron crystals is so striking in comparison to both individual submicron crystals and currently commercially employed multimicron crystal/aggregates that zeolites in this physical form may well be viewed as a new composition of matter. Zeolites including zeolite A, zeolite X and high aluminum analogs, as well as high aluminum analogs of gismondine, e.g. zeolite B, P and MAP, among others can be provided with the novel particulate morphology of this invention.
The synthesis of zeolite A has been well established owing to its wide employment as an ion-exchange agent, especially useful in water softening applications such as detergent building. With a Si/Al ratio of 1.00, the lowest possible for a zeolite according to the Rule of Lowenstein, high aluminum environments have generally been used in manufacture. The employment of a silica-enriched environment is counter-intuitive for the synthesis of such high aluminum materials. However, the employment of a silica-enriched environment instills several properties in a synthesis mixture. First, it allows the addition of substantial hydroxide and/or other electrolytes beyond that which would be applicable to a xe2x80x9cnormalxe2x80x9d zeolite synthesis procedure. Such enhanced alkalinity/electrolyte levels are well known to promote the nucleation of zeolite seed crystals. Secondly, silica-enhanced environments promote retention of macroscopic particle integrity in the synthesis of aluminosilicates. These two properties, enhanced nucleation and retention of particle integrity, lead to the formation of massive numbers of submicron crystals of the zeolite forming as macroscopic aggregates, especially when a solid source of aluminum is employed, such as metakaolin and/or alumina.
The synthesis environment of the hybrid zeolite-silica compositions as described in aforementioned U.S. Pat. No. 5,948,383 has been found to promote the formation of aggregates of submicron HZSC crystals. Such compound aggregates manifest the rapid exchange properties of submicron crystals and the favorable handling properties of macroscopic aggregates. This favorable set of properties results from the high alkalinity of the HZSC synthesis mixture (versus comparable zeolite synthesis) in combination with the propensity of aggregates to retain mechanical integrity in silica-rich environments. It has been further discovered that the elevation of the electrolyte concentration in HZSC synthesis mixtures beyond that inherent to the caustic necessary for synthesis instills even more enhancement in the water softening properties of such materials. This elevation in electrolyte concentration may be accomplished by the addition of soluble salts to the synthesis mixture. Such salts include Na2CO3, TSPP, Na2B4O7, as well as ordinary table salt (NaCl). Such salt addition causes the formation of smaller submicron crystals (as manifested by increased exterior surface area) without reduction of aggregate particle size (typically 1-5 microns). The superior water softening properties are especially observed in the enhanced rate of ion-exchange kinetics. This effect has been demonstrated for both zeolite P and zeolite A HZSC-analogs and is most likely applicable to other HZSC materials.
An additional advantage of the intentionally elevated electrolyte level in HZSC synthesis mixtures is in the enhanced rate of reaction during zeolite formation. As a consequence shortened reaction times and/or reduction in reaction temperatures, may favorably impact the economics of HZSC manufacture. While this effect has been demonstrated for HZSC, it is likely that it is directly applicable to classical zeolite synthesis. Thus, it would be expected that crystals of zeolite (for example zeolite A) grown at elevated electrolyte levels would be smaller than those grown under equivalent conditions without salt addition. For the synthesis of in situ zeolite particles, this might well result in an alternative means to prepare macroscopic aggregates of microcrystalline zeolites.